The present invention relates to a semiconductor device, and more particularly to an improved field effect transistor (FET) which comprises a compound semiconductor having a heterojunction and can operate at a high speed by using a two-dimensional electron gas (2DEG), and a method of producing the device.
A FET having a heterojunction and utilizing a 2DEG is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 57-7165, 57-176773, and 57-180186 and European patent application No. 82302107.6, i.e., EP-A No. 0064370. A main characteristic of the FET is high electron mobility due to accumulation of a 2DEG at the heterojunction interface and decrease of the ionized-impurity scattering effect. As the temperature of the FET is reduced, the lattice scattering effect is reduced and the mobility is further increased.
With a FET of the gallium-arsenide (GaAs) type, however, there is a problem that the parasitic source resistance Rs and the parasitic drain resistance Rd are relatively large, since, mainly, an aluminum-gallium-arsenide (AlGaAs) layer having a relatively high resistivity exists between a GaAs layer and the source and drain electrodes.
To improve the properties of the FET, it is necessary to decrease the source resistance Rs and drain resistance Rd. Accordingly, the source electrode and drain electrode are made of gold-germanium (AuGe) and are heated so that the AlGaAs layer and GaAs layer alloy with the AuGe. Thus, ohmic contact regions including a source region and a drain region and having a relatively low resistivity are formed under the electrodes. The contact resistance between the alloyed regions and the 2DEG layer is still, however, relatively large.